Grease spot photometer



1956 G. D. CREELMAN 2,771,000

GREASE SPOT PHOTOMETER Filed May 19, 1954 lllll FIG I INVENTOR. GEORGE D. CREELMAN 055mm dZZWg/J.

ATTORNEY United States Patent GREASE SPOT PHOTOMETER George D. Creelman, Cleveland Heights, Ohio Application May 19, 1954, Serial No. 430,809

I 3 Claims. or. 88-23) This invention relates to a photometer and, more particularly, to an improved grease spot photometer which can convert the light intensity from any source to the proper aperture for a camera using film of a given sensitivity.

In photography the amount of light that reaches the light-sensitive film is determined (1) by shutter speed or the time the film is exposed, (2) by the aperture or crosssectional area of the camera diaphragm, (3) by the type of light-filter, if any, which might be used, and (4) by the intensity of light comingfrom the subject to be photographed. For practical purposes, the nature of the subject determines shutter speed and the type of light filter to be used. Variations in light intensity are compensated by aperture adjustments and, therefore, the problem of most photographers is to estimate light intensity accurately.

If the amount of light reaching a film is too great, the

film is said to be overexposed. Conversely, if too little light reaches the film, it will be underexposed. When using ordinary black and white film, reasonably clear pictures are obtainable when the exposure varies from one-half to double the ideal value. This variation is compensated by the comparatively low sensitivity or wide latitude of the film. Few people are able to estimate exposures within the latitude available for black and white films. Their problem is more acute in color photography because the latitude of such film only permits a variation of 0.75 to 1.5 times the ideal value. Therefore, commercial, as well as amateur, photographers have come to rely on various types of light meters to measure the intensity of light coming from any subject.

There are two common types of light meters, both of a whose opacity is graduated. The point on the strip at which light is just extinct is correlated with the correct camera aperture. This point is determined by the eye. But in view of the well known fact that the human eye adjusts itself to illumination by enlarging or contracting its pupil, the judgment of the eye is unreliable.

. An advantage of my invention is that it provides a light meter which is accurate, inexpensive, compact, and independent of the sensitivity of the eyeto illumination.

. Another advantage is that it is more sensitive in dim light than all but the most expensive photoelectric meters.

A still further advantage is that its preferred embodiment has no moving parts.

My invention makes use of the principle of the comparative or grease spot photometer. For example, a spot e 0 2,771,000 ICC Patented Nov. 20, 1956 of grease or wax on paper will appear dark or light as "compared with the adjacent unwaxed paper, depending on whether the intensity of light reflected from the viewed surface adjacent the spot is greater or less than that transmitted through the spot from its reverse side. This spot is quite sensitive to small. dilferences in illumination in that, for example, when disposed between two lights it will change from dark to light as a result of a very small decrease in the illumination on the adjacent viewedsurface or a corresponding increase on its other side. The change is independent of the sensitivity of the eye.

- The nature and scope of my invention will be better understood from a consideration of the following discussion and accompanying drawings in which:

Figure 1 is a plan view of the preferred embodiment of my photometer, and

Figure 2 shows the details of an indicator strip.

Referring now to Figure 1, my photometer consists of a pocket size container or box 11, preferably painted black on the inside, with an eye opening or window 12 in one end and a view opening or window 13 on the opposite end. A lens 14 and a diffuser plate 15 may be fixed just inside the eye and view openings 12 and 13, respectively. a

The use of a lens is preferred, and it should have a magnifying power great enough so that one using my photometer can focus his eye clearly on the grease-spot indicator strip 16 which in most cases will be closer than the normal eye can focus. I

The Flys Eye type diffuser, as illustrated, is preferred because it will reflect unusually bright light coming from an angle. Such light would otherwise introduce some error in the result. If a flat or ground glass diffuser is employed it should be recessed deeper into the case 11 away from the opening 13. Ground glass is preferred over opal or milk glass for diffusers. A

The grease-spot indicator strip 16 is disposed at an angle to the line of vision between the two openings and is held by suitable clamps '17 and 18. It completely partitions the case 11 between the two openings. A light bulb 19 is afiixed to the divider 21 at the end of the case 11 in which is located the eye opening 12. Direct illumination of the opening 12 by the bulb 19 is prevented by the bafiie 20. A divider 21 provides space in the case 11 for the dry cell batteries 22, preferably of the mercury type used in hearing aids because of their longer life. The bulb 19 is lighted by pressing the spring loaded switch 23. The batteries are thereby forced against the contact element 24 which in turn completes an electrical circuit by touching the bulb 19. Y

The details of the grease-spot indicator strip 16 are shown in Figure 2. The strip should contain two sections whose relative brightness will be compared. One section, such as shown in the shaded portion 30, must be translucent. If the other section is also translucent, it must be of a different degree; otherwise, no comparison could be made.

A typical embodimentwould be a strip of white bond paper saturated with a low melting point paraflin wax, grease, or some other light and relatively clear hydrocarbons. Ordinary cigarette lighter fiuid (petroleum naphtha) may be used although his not recommended. This saturant may be applied to the strip as shown-in the shaded or translucent portion 30. Another embodiment of the indicator strip might consist of a strip of polyethylene whose shaded portion 30 is translucent and the balance opaque. The shaded portion 30 isdivided into a plurality of sections which are numbered from one end to the other. The numbers, which are placed above the shaded sections, correspond to aperture settings normally found in present-day cameras.

Theactual correlation. betweenthese. numbersand the correct aperture for any particular film may be placed in a. table to be provided with the photometer. A number-rangetrom onetoiten; for example; wouldcorrespondtoaperture SfliIigShOf.114,12,J2;8u.4j15,698, 1 13,16, 2246; and32 fora film having amexposurezratingor-A.: S; E. Index of: 50: If theexposure times oft allzfilmsr were the same,.th'e table would; not beinecessary. and: the actual aperture setting numbers could .besplaeedonithe strip as shown in FigUreaZ.

In place of. the table it wouldbe possible to provide a plurality of indicator strips with. each case and the viewer would-select; and use the one: which correspondsto: the exposure time of the film he proposes to.use.in:his:camerai Inoperation the viewer'wou'ld placehiseyeat theopeningqlziandipointithe casexll toward thexsnbjectsto be photographed. Thee bulb 19xshould-then be'lightedrby pressing the switclrZSi- Heca'nzthen determinethe point on the indicator strip 16 where the saturated portion appears to beaszbrightasxthe adjacent .unsaturatediportion. The number above this point is the. correct aperture 1 setting.

The. use of thediffuser 15.-is preferred because: it averagestheillumination. received. This averaged illumination then passes-tocthe indicator strip 16. If the diifuser were omitted and the subject to be photographed containecba small-but intense light, the observed aperture setting would not be as accurate as the use of some films might:require.-.

While notrequired, the presence of the lens 14 is pre-. ferred: It:permits the useof asmaller opening 12 and enables one to make. more accurate. readings because the indicator strip will be in clear focus.

Thetrange of, my photometer is in large. part dependent on the positionof the indicatorstrip 16 inside the case '11. Asumentioned before, this comparative type photometer uses aifla'sh' light .bulb powered by small dry cell batteries as a .lightsou-rce. The intensity. of lighter illumination from this source on the indicator strip must vary along the strip so that it may. becompared with the: amount-of lightifalling. on the othenside. This variation inillumifiation is produced by placingthe stripatan'angle with the line otisightas shownin Fig. .1. The light falling on the strip; from-the bulb 'variesi directly. with thesine of the angle of'incidence A and inversely with the squareof thetdistance' D of thestripfromthe bulb- Stated as a formula, the relation'is:

Illumination (I) gr -i K is a-constant; At clamp 17it will be evident that I is ata-maximum because Dis at a minimum and sin A is at amaximum; If in operation theindicator-changes from dark to-light inthe/region near clamp 17, the illumination from the subject to be photographed will'be very'large. The corresponding aperture will be quite small.

Atsome point *on'thestrip remote from'clamp 17, angle A" will be" much smaller andD correspondingly greater; Obviously, therefore,- illumination will be quite' small; comparatively, and the aperture large.

The" scale of the-indicator stripmay be increased by curving it away from the bulb 19 and particularly curv ingthat portiomofthe strip near the clamp 1'8. In effect this curvature .would lower the angle" A thereby di minishing the illuminationinthat region. The useful rangetof my photometer may: also be extended by placingatlight filter in frontof the bulb 19.-

In the final stagesof: manufacturing the embodiment showntin" Figure 1, it may be necessary to adjust the position of the indicator along the line of vision to-account for normal variations in light bulb and/or battery outputs.

From the foregoing-discussion itwill be' seen that the comparison of illumination from the standard bulb and from the subject to be photographed is accomplished in the embodiment shown in Figure 1 by varying their relative distance from the indicator strip. By appropriate modifications the same result can be attained by two other methods.

The first alternative embodiment may be constructed by replacing the diffuser with a light valve in the form of a graduated filter or optical wedgeby means of which the intensity of light transmitted to the indicator. strip 16 (placed perpendicular to the line of vision) is varied to match the light from the bulb 19; A.graduated filter strip, for example, may be drawn across the opening 13 by winding it up on a small drum. The position of the drum would be correlated with aperture settings.

In the second alternative embodiment the intensity of the light from the bulb which reaches the indicator strip (again: placed perpendicular to the line of vision) would be varied by incorporating a variablerheostat in the electrical. circuit. Aperture settings would correspond to the rheostat position.

The embodiment shown in Figure 1 is preferred over the two alternates described because it contains nornoving parts.

Foraccurate results, particularly. when using. color film; it maybe necessary from time to time to obtain an absolute calibration. of my photometer by taking a reading at a measured distancefroma standard incandescent. bulb, or bycomparing its. results with a sensitive photoelectric meter, and applyin-g'appropriate corrective'factorsto the indicator strip settings.

Anyl errors which develop will be caused primarily by the slow-deterioration of the dry cell batteries. When their useful life is about gone, their voltage begins to drop at an increasingrate; Thisvoltage drop affects the amount of-light emitted by thebulb. Since this light is proportional to the square of the voltage, a voltage drop from /2 to 1', for example, will result'in a drop in the amount'ofdight from 2 to l. A 50% drop in the amount of light wouldv require a doubling of the aperture. The latitudeof ordinary black and white film, as mentioned above, would not permit errors greater than this.

I'have described only a few of the many embodiments of my invention which willoccur to those skilled in the 3T1;

1. A portable photometer comprising a box with. a window in the first of two opposite sides for admitting the light to be measured, an aperture in the second oppositeside-for observing the admitted light, a light source near onecorner-of the box adjacent the aperture, a light shieldbetween the. light source andthe aperture, and a vertical partitionin the box between the window and aperture comprising an indicator strip positioned opposite said corner and at an acute angle with the line ofsiglit'between the windowand aperture;

2.- The'device of claim 1 which contains a ditfuser plateover the second window.

3'. Thedevice of claim 2 in which the light is powered by meansof at least one dry cell battery.

References Cited inthe file of this patent UNITED. STATES PATENTSv Great'Britain Jan; 13, 1927 

